Various types of keyswitches are in use. Such keyswitches include a keytop for manually actuating a switch mechanism. A particularly reliable type of keyswitch is a capacitive keyswitch in which the capacitance changes in response to depression of the keytop. When used to construct a keyboard, the keyswitches are arranged in a matrix array and continually scanned to sample the condition of each individual keyswitch. In response to a depressed keyswitch, a decoder generates a coded electrical signal which is a representation of the actuated keyswitch.
Frequently, such decoder circuitry is provided with a feature called N-Key rollover. A keyboard is said to have N-Key rollover when several keyswitches may be depressed in succession with the preceding keyswitches maintained in the depressed state. The decoding circuitry, having an N-Key rollover feature, generates each corresponding keyswitch signal in the proper sequence once for each keyswitch depressed. Prior circuits for sensing and processing capacitive keyswitch keyboards have exhibited difficulty in providing an N-Key rollover feature. In such keyboards, if several keyswitches are maintained closed at the same time, there are unwanted mutliple capacitive paths between the various rows and columns comprising the keyboard matrix which may couple a signal across one or more non-depressed capacitive keyswitches. In such arrangements, the undesired capacitive coupling paths generate extraneous signals and the number of such extraneous signals is directly related to the number of keyswitches being simultaneously held in their depressed condition. This situation prevents the inclusion of a reliable N-key rollover feature in prior capacitive keyswitch keyboards.